Barnes et al in U.S. Pat. Nos. 2,975,603; 3,086,370; and 3,217,503 disclose processes for preparing ice products containing from about 25 to about 120 milliliters of carbon dioxide, or other suitable conditionally-stable-hydrate-forming gas, per gram of frozen product. According to one aspect of these related disclosures, carbonated ice was prepared by subjecting aqueous liquid to a carbon dioxide pressure of at least about 200 psig and preferably less than 600 psig; maintaining the aqueous liquid and the carbon dioxide in contact for a time sufficient to permit absorption in the liquid of carbon dioxide in bound form; and freezing the reaction mixture which contained carbon dioxide hydrate crystals suspended within unreacted aqueous liquid.
Alder et al, in U.S. Pat. No. 3,220,204, state that while the prior art procedures of Barnes et al produced products which would retain a high level of carbonation during frozen storage, the products had a tendency to explode or pop during dissolution of the product to release the gas. Alder et al indicate that when the Barnes et al carbonated ice products were added to water or milk, they would frequently explode in the glass. To correct this, Alder et al subjected a thin film of water to carbon dioxide gas at a pressure and temperature above the eutectic point of the water, the temperature being low enough to form a hydrate. They stated that, as a practical matter, in order to operate under controllable conditions, hydrate should be produced at a pressure above 200 psig and at a temperature above 0.degree. C., in order to maximize hydrate formation while minimizing collateral formation of water ice. After suitable hydrate formation, the reaction mixture containing water and hydrate crystals was frozen at a temperature below -3.degree. C.
In U.S. Pat. No. 3,255,600 to Mitchell et al, there is disclosed a process for forming carbonated ice wherein liquid carbon dioxide and liquid water or water ice are mixed under controlled conditions. The patentees indicate that they discovered that liquid carbon dioxide results in a more rapid formation of the product while permitting more accurate control of the operating conditions. It has been our experience, however, that the use of liquid carbon dioxide requires the use of great quantities of energy and produces a product which loses significant gas content before it can be commercially distributed; and it has the popping and cracking problems associated with the earlier prior art.
Throughout this evolution of gasified ice products involving reactions above the freezing point of water, Mitchell et al disclose in U.S. Pat. No. 3,333,969, that the problem of uneven release of other gas had persisted. Mitchell et al focused on a method for subdividing carbonated ice into discrete particles while maintaining the temperature of the ice below 0.degree. C., and then compacting the discrete particles to form them into an adhered mass or briquette to eliminate the explosive release of carbon dioxide during carbonation. This process actually resulted in a decrease in final gas content.
In a departure from the above techniques, which carry out the content between the conditionally-stable-hydrate-forming gas and the water at temperatures above 0.degree. C., it is disclosed in co-pending U.S. patent application, Ser. No. 326,888 entitled, "Carbonated Ice Product and Process," filed by Hinman et al. concurrently with the present application, that a carbonated ice product could be efficiently prepared by contacting water ice with carbon dioxide at a temperature below 0.degree. C. That application discloses the discovery that when the conditions of contact are controlled to maintain the water ice in the frozen state and the carbon dioxide in the gaseous state, a commercially satisfactory rate of reaction is noted at temperatures just below the freezing point of water. That discovery for the first time makes it possible to practically utilize the product of the reaction between water ice and a gaseous conditionally-stable-hydrate-forming gas, for utilization in a commercial process. Prior to that time, Miller and Smythe in Science, Vol. 170, October 1970, pages 531-533, disclosed the formation of a carbon dioxide hydrate by a gas-solid process at temperatures of from -73.degree. to -43.degree. C. as part of their study of the kinetics of decomposition at temperatures of from -121.degree. to -101.degree. C. Miller and Symthe determined a temperature-dependent decomposition rate which, if extrapolated to the temperature range normally encountered during commercial and home freezer storage, would indicate an entirely unsatisfactorily high rate of decomposition. The results of Miller and Smythe were later found to be consistent with the work of Adamson and Jones in a study published in the Journal of Colloid and Interface Science, Vol. 37, No. 4, December 1971, pages 831-835. Adamson and Jones also dealt with temperatures of less than -73.degree. C. Thus, the work disclosed by Hinman et al. in the above-identified co-pending patent application was surprising in that the rate of reaction dramatically increased within a narrow temperature range just below 0.degree. C. However, the Hinman et al. product is less stable than would be desired for a commerical product, especially one intended for distribution over a large geographical area for which an extended shelf life would be necessary.
It is apparent from the foregoing discussion of the prior art that the problem of providing a gasified ice product having a shelf life suitable for commercial distribution in the frozen state, and an even evolution of gas upon melting, have been significant concerns. For gasified ice products prepared by techniques other than gas-solid contact, significant progress has been made addressing these concerns. However, gasified ice products prepared by gas-solid contact are presently in need of significant improvement, especially in the area of storage stability.